Examples of gases containing oxides of nitrogen (hereinafter referred to as NO) are exhaust gases from combustion apparatuses such as boilers, nitric acid manufacturing plant, various metal treating processes and other nitrogen oxide generating plants.
In recent years, it is known that a so-called photochemical smog is generated frequently. One of the main causes of such photochemical smog is that a large quantity of NO is present in the atmosphere. It is therefore much expected to reduce the quantity of NO contained in such exhaust gases and/or to remove NO from such exhaust gases.
In combustion apparatuses such as boilers, for example, the NO content in the exhaust gas has been reduced conventionally by employment of the burners and furnaces of improved design. These methods, however, are not very desirable because they allow the reduction of NO only within narrow limits for both the theoretical and economical reasons.
From the thesis of J. W. Mellor ("A Comprehensive Treatise on Inorganic and Theoretical Chemistry," Vol. VIII, 1931, p.423 -) and thesis of B. F. G. Johnson and J. A. McCleverty ("Progress In Inorganic Chemistry," Vol. 7, 1966, p.277 -), it is known for long that salts of certain metals react with NO to form complexes. As is discussed by S. N. Gang and L. I. Mamon in "Journal of Applied Chemistry, U.S.S.R." 26, 1953, p.925 -, it is already known to absorb and remove NO by utilizing above-mentioned properties of metal salts.
A conventionally proposed method for removing NO from a gas containing NO is to bring said gas into contact with an aqueous solution containing said metal salts. Such method, however, is not very effective for removing NO.sub.x from a large quantity of exhaust smoke or exhaust gas. Because, even when a ferrous salt which, of all said metal salts, seems to have the highest ability of forming complexes with NO is employed, the gas/liquid contact has to be effected for a long time to remove NO which is present at a low concentration by reason that the NO absorption quantity per unit liquid quantity is extremely small.
Thus, any effective method for removing NO from a gas containing NO has not been proposed by now.